Capacitor banks, also called shunt capacitor banks, denoted by CB/SCB(s), are installed to improve the quality of an electrical supply and to provide capacitive reactive compensation and power factor correction in a power system by shunting away and concealing current fluctuations from a primary power source. The use of capacitor banks has increased because they are relatively inexpensive, easy and quick to install, and can be deployed almost anywhere in a power grid system. Capacitor bank installations have other beneficial effects on the system such as improvement of the voltage profile, better voltage regulation, reduction of losses and reduction or postponement of investments in the transmission and generation capacity.
A capacitor bank is assembled by a plurality of individual capacitor units. Each individual capacitor unit is a basic building block of a capacitor bank and made up of individual capacitor elements, arranged in parallel/series connected groups, within a steel enclosure. The internal discharge resistor is also included in order to reduce the unit residual voltage after being disconnected from the power system. The capacitor unit used in the power grid system normally includes aluminum foil, paper, or film-insulated cells immersed in a biodegradable insulating fluid and are cased in a metallic container.
Capacitor banks are normally constructed using individual capacitor units connected in series and/or parallel to obtain the required voltage and mega unit of volt amperes reactive ratings, denoted by MVAr. Typically the neighboring capacitor units are installed in racks. Each rack is insulated from each other by insulators.
Outings of a capacitor bank are often caused by accidental contact by animals. Vermin, rodents, cats, birds etc. use the capacitor banks as a resting place or a landing site. When the animal touches the live parts, a flash-over occurs that may result in unnecessary interruptions or consequential damages to the whole bank, unless the bank is sufficiently fitted with protection relays. Moreover, the design of a capacitor bank is adapted to avoid cascading failures although they cannot be completely ruled out.
A cascading failure is a type of series fault, which means that a failure in a capacitor unit results in another failure in a neighboring capacitor unit. Thus, a conventional capacitor bank differential protection will not protect against such failures. When a cascading failure occurs, a fault current drawn from the power grid system can be limited by the impedance of the remaining healthy parts of the capacitor bank, which results that the failure will not be detected by a standard capacitor bank over-current protection until the failure evolves to a number of at least 30-50% of connected units.
An unbalance protection which is typically available within capacitor bank protection scheme is designed to detect element failure within a unit. It however often reacts too slow and/or can even be completely disarmed by quite high level of the unbalance quantity caused by a cascading failure between two capacitor units or two capacitor racks.
Failure to provide adequate protection for such series unbalance fault within the shunt capacitor bank may lead to one or more of the following situations: excessive damage to the capacitor bank; adverse system effects; spread of damage to adjacent equipment; excessive period when the damaged equipment is unavailable; and possible capacitor unit casing rupture and undesirable discharge of dielectric liquid and/or fire.
Therefore, a protection scheme against a cascading failure between two neighboring capacitor units/racks is highly desired.